Missy is done. Gomez said she was done with it. Although, Moffat did leave a few possible "outs" in the series 10 finale. 1. Missy somehow survived, 2. Missy survived by Regenerating, or 3. There was another incarnation between Simm and Gomez. So it is open for Chibnall to explore if he chooses.
Personally, I feel we may get the third option. However I hope the new production teams steers clear of The Master for a year or two. Same with the Cybermen (who are my favourite monsters.) Now, if Moffat was still involved, I could see him doing option #3. But he would complicate it by having the new Master face the 13th Doctor, only to have them Regenerate into Missy. Inspired by the fact the Doctor became a female. So, naturally, the Master would copy him.